1. Field of the Invention
This invention relates to a semiconductor integrated circuit device, and in particular to a semiconductor integrated circuit device which generates and distributes a synchronous clock signal.
2. Description of the Related Art
In LSI for microprocessors using a CMOS circuit, a clock signal is distributed to an internal CMOS circuit, and operations are performed according to the timing of the clock signal. In general, a phase-locked loop (PLL) is built into the LSI as a circuit for generating the clock signal. The clock signal is generated by a crystal oscillator circuit, etc., outside the LSI. The signal is then input to the LSI, the PLL performs frequency multiplication and phase synchronization, and the clock signal is distributed to the LSI internal circuit.
One example of a microprocessor using a PLL is the system described in xe2x80x9cHitachi SuperH RISC engine SH7750 Hardware Manualxe2x80x9d pp. 10-1 to pp. 10-15, published in April 1998. A block diagram of this prior art example is shown in FIG. 28. The clock signal generated by the crystal oscillator circuit is input from terminals xta1 and exta1 of an oscillator OSC, divided by a frequency divider DIV1, and transmitted to a PLL PLL1. In the PLL PLL1, the frequency of the input clock signal is multiplied by a factor of, e.g., 6 times, and is output. This multiplied clock signal is shaped into clock signals of desired frequencies through a frequency divider DIV2, and is distributed to the LSI internal circuit as plural clock signals. These clock signals are further supplied to a PLL PLL2, phase-synchronized, and output to the exterior of the LSI from a terminal ckio.
A prior art example of a PLL is shown in FIG. 26. An input clock signal clkin and output clock signal clkout which have been divided by a frequency divider DIV00 are input to a phase frequency detector PFD, and their frequency difference and phase difference are detected. The detected frequency difference and phase difference are converted into a control voltage by a charge pump CP and low pass filter circuit LPF, and are used as a control signal of a voltage control oscillator circuit VCO. The voltage control oscillator circuit VCO outputs a clock signal as the output clock signal clkout according to the control voltage, and feeds it back to the phase frequency detector PFD through the frequency divider DIV00. By repeating this process, the PLL generates the output clock signal clkout which is phase-synchronized with the input clock signal clkin and having a multiplied frequency. The time until the clock signal output by the PLL can be supplied stably with the desired phase and frequency is referred to as the clock settling time. For example, in the example described on pp. 346 to 347 of the 1999 IEEE International Solid-State Circuits Conference Digest of Technical Papers (February 1999), the clock settling time of the PLL is 40 periods of the clock signal.
In CMOS LSI used in microprocessors for portable devices in recent years, attempts are being made to reduce power consumption by such means as suspending supply of the clock signal to the circuit when the LSI internal circuit is not operating. Such an LSI has three kinds of states. One is an active state which performs normal operation. The second state is a sleep state where only the clock generating circuit is operating, the internal circuit is halted and the clock is not supplied to the internal circuit. The third state is a standby state where all circuits in the LSI are halted. In the standby state, the clock generating circuit is also stopped, and when the state changes from standby to active, the internal circuit cannot start operating and a fast return to the active state cannot be performed until the clock signal generated by the clock generating circuit stabilizes, i.e., during the clock stability period. Hence, in the sleep state, only the internal circuit is stopped and the clock generating circuit is made to function. When there is a state change from the sleep state to the active state, as the clock signal can be supplied stably, there is a fast return to the active state. However, the power consumption is larger than in the standby state due to operation of the clock generating circuit. The reason why a fast return to the active state from the standby state is not possible, is that the clock settling time of the PLL which is the clock generating circuit, is 40 periods of the clock signal, as described on p. 346 to p. 347 of the 1999 IEEE International Solid-State Circuit Conference Digest of Technical Papers (February 1999). If a clock generating circuit having a short clock settling time of 2-3 periods can be used for the clock generating circuit in a microprocessor, a fast return to the active state would be possible even if operation of the clock generating circuit stops in the sleep state.
It is therefore an object of this invention to provide a semiconductor integrated circuit device comprising a clock generating circuit having a short clock settling time, thereby resolving the above-mentioned problem.
This invention therefore provides a semiconductor integrated circuit device comprising plural clock generating circuits. As the clock settling time of the clock generating circuits is as short as 2-3 periods, the clock generating circuits stop simultaneously when the internal CMOS circuit of the semiconductor integrated circuit device stops, thereby reducing power consumption, and the internal CMOS circuit can be returned to the normal operating state from the idle state at high speed.
To resolve the above-mentioned problem, the main features of this invention are as follows.
The oscillator circuit inside the CMOS LSI circuit comprises a crystal oscillator, selectors, frequency dividers, and a phase-locked loop (PLL) and delay-locked loop (DLL).
An oscillator OSC comprises a terminal which receives a signal from outside the LSI. The signal input to this terminal is input to the crystal oscillator. The clock signal output of the crystal oscillator and the external clock signal directly applied from the terminal are selected by the selector. The output selection of the selector is also performed by a terminal. Next, the clock signal is divided by a frequency divider. Whether the clock signal is transmitted directly or whether the division output signal of the frequency divider is transmitted, is selected by a selector. Next, the clock signal is supplied as a reference signal to the PLL and DLL, and synchronized clock signals are respectively generated. Whether the clock signal is transmitted directly, or whether the clock signal output by the PLL or the clock signal output by the DLL is transmitted, is selected by a selector. The clock signal is then divided by a frequency divider, and distributed to the LSI as clock signals. The signal divided by the frequency divider is obtained as clock signals of desired frequencies by plural selectors, and clock signals having plural, different frequencies are supplied to the LSI.
According to another embodiment of this invention, a clock control circuit in a CMOS LSI circuit comprises a crystal oscillator, selectors, frequency dividers, PLL and DLL, and a clock control circuit in a CMOS LSI circuit comprises a frequency control register and bus interface.
The oscillator has the same construction as that of the above-mentioned circuit. The clock control circuit receives a clock control command from the internal bus of the LSI circuit, a frequency control signal is output to determine the output selection of a selector in the oscillator, and the PLL or DLL and division ratio of the frequency divider are selected. In the clock control circuit, the bus interface decodes the clock control command from the internal bus, and the frequency control register is rewritten. A frequency control signal is output according to the setting of the frequency control register, and a selector of the oscillator is selected.
According to another embodiment of this invention, a clock control circuit inside a CMOS LSI circuit comprises a crystal oscillator, selectors, frequency dividers, PLL and DLL, and a clock control circuit inside the CMOS LSI circuit comprises a frequency control register.
The oscillator has the same construction as that of the above-mentioned circuit. The clock control circuit has at least one mode terminal which receives a signal from outside the LSI circuit. A frequency control register is determined by the input signal of the mode terminal. An output is generated according to the frequency control register setting.
According to another embodiment of this invention, an oscillator inside a CMOS LSI circuit comprises a crystal oscillator, selectors, frequency dividers, PLL and DLL.
The oscillator OSC has a terminal which receives a signal from outside the LSI, and the signal input to the terminal is input to a crystal oscillator. The clock signal output of the crystal oscillator and an external clock signal directly supplied from the terminal are selected by a selector. The selection of the selector is performed by an external signal input to the terminal. Next, the clock signal is divided by a frequency divider. Whether the clock signal is transmitted directly or whether the division output signal of the frequency divider is transmitted, is selected by a selector. Next, the clock signal is supplied as a reference signal to the PLL and DLL, and synchronized clock signals are respectively generated. Whether the clock signal is transmitted directly, or whether the clock signal output by the PLL or the clock signal output by the DLL is transmitted, is selected by a selector. The clock signal selected here is divided by the frequency divider, and distributed to the LSI as clock signals. The signal divided by the frequency divider is obtained as clock signals of desired frequencies by plural selectors, and clock signals having plural, different frequencies are thus supplied to the LSI. One of the clock signals supplied to the LSI is also supplied as a reference signal to the PLL and DLL, and synchronized clock signals are respectively generated. Whether the clock signal is transmitted directly, or whether the clock signal output by the PLL or the clock signal output by the DLL is transmitted, is selected by a selector. The clock signal selected here is output from a terminal to outside the LSI circuit.
According to another embodiment of this invention, an oscillator circuit, clock control circuit and timer circuit are provided inside a CMOS LSI circuit. The oscillator has the same construction as the above-mentioned circuit. The oscillator inputs a signal from outside via a terminal, inputs a frequency control signal and a clock standby signal from the clock control circuit, outputs a system clock signal to outside the LSI circuit using a terminal, and outputs one or more clock signals to the LSI circuit. The clock control circuit has one or more mode terminals, inputs a mode control signal from outside, inputs an interrupt signal, reset signal and standby enable signal from the LSI circuit, receives a control command from an internal bus, inputs a standby cancel signal from the timer circuit, and outputs a frequency control signal and clock standby signal to the oscillator and timer circuit. The timer circuit receives a control command from the internal bus, inputs the clock signal from the oscillator, inputs the frequency control signal and clock standby signal from the clock control circuit, and outputs the standby cancel signal to the LSI circuit to the clock control circuit.